1. Field of the Invention
The present invention relates to a method of producing a fluororesin aqueous dispersion.
2. Description of the Prior Art
Polytetrafluoroethylene-based fluororesin aqueous dispersions, when applied in the manner of coating or dipping, for instance, can form films or layers excellent in such characteristics as chemical stability, nonstickiness and weather resistance and, therefore, are widely used in such fields of application as cooking utensils, pipe linings and impregnated glass cloth membranes. In these fields of application, fluororesin aqueous dispersions having a high fluororesin concentration are generally preferred, so that those products obtained by polymerizing a fluoromonomer(s) in an aqueous medium in the presence of a fluorine-containing surfactant, followed by concentration are in current use. From the cost viewpoint, however, it is desirable that fluororesin aqueous dispersions be deprived of such a fluorine-containing surfactant.
For fluorosurfactant removal from fluororesin aqueous dispersions, a method has been proposed which comprises repeating a separation operation in the presence of a nonionic surfactant and an electrolyte substantially three times, each time separating the supernatant and recovering the lower phase (cf. e.g. Patent Document 1: Japanese Kokai (Laid-open) Publication 2005-126715 and Patent Document 2: Japanese Kokai Publication 2005-171250). However, this method requires substantially three repetitions of such concentration procedure.
In Patent Document 3 (United States Patent Application Publication 2004/186219) and Patent Document 4 (United States Patent Application Publication 2004/171736), there is described a fluororesin aqueous dispersion reduced in the content of ammonium perfluorooctanoate [PFOA] by means of an anion-exchange resin and further reduced in viscosity by addition of a fluorine-free anionic surfactant in an amount within the range of 1 to 12% relative to the fluororesin. However, these documents have no mention of the technique of concentration through phase separation.